Document the behavior of Robot::getPixelColor with HiDPI

That is, confirm that this method will return the real "pixel" color at point x,y on the HiDPI screen. On a HiDPI screen the unit at point (x,y) may have 4 pixels, and it is unclear, which color will be returned:

1.) The color which interpolate 4 pixels
2.) The color of the top/left pixel.

Once confirmed, this should be documented.